1. Field of the Invention
This invention relates to a backlight apparatus, a backlight controlling method and a liquid crystal display apparatus.
2. Description of the Related Art
A liquid crystal display (LCD) apparatus generally includes a color filter substrate which is colored in red (hereinafter referred to simply as R), green (hereinafter referred to simply as G) and blue (hereinafter referred to simply as B), a liquid crystal panel having a liquid crystal layer, and a backlight disposed on the back face side of the liquid crystal panel.
In the liquid crystal display apparatus, the twist of liquid crystal molecules of the liquid crystal layer is controlled by changing the voltage. Then, when white light of the backlight passing through the liquid crystal layer passes through the color filter of R, G or B in response to the twist of the liquid crystal molecules, it is converted into R, G or B light thereby to display an image.
It is to be noted that to change the voltage to control the twist of liquid crystal molecules to change the transmission factor of light is hereinafter referred to as control of the liquid crystal transmission factor. Further, the luminance of light emitted from the backlight as a light source is hereinafter referred to as “light emission luminance”, and the luminance of light emitted from the front face of the liquid crystal panel which is the intensity of light felt by a viewer who looks at the image displayed on the liquid crystal display apparatus is hereinafter referred to as “display luminance”.
In the past, for example, a cold cathode fluorescence discharge lamp (hereinafter referred to as CCFL) or a white LED (Light Emitting Diode) has been used as a light source of a backlight for a liquid crystal display apparatus. A liquid crystal display apparatus to which a CCFL or a white LED is applied as a light source of the backlight has a problem that the reproduction range of color represented by the area of a triangle formed from three apexes of R, G and B on an xy chromaticity diagram according to the CIE (Commission International de l'Eclairage) 1931 calorimetric system is smaller than that of a CRT (Cathode Ray Tube) display unit.
As a countermeasure against this problem, a liquid crystal display apparatus wherein R, G and B LEDs are adopted as the light source of the backlight to expand the reproduction range of color is disclosed, for example, in Japanese Patent No. 3766042 (hereinafter referred to as Patent Document 1).
In the liquid crystal display apparatus disclosed in Patent Document 1, the backlight includes a plurality of LED blocks, each of which is formed from R, G, and B LEDs. In each LED block, the current values of current to be supplied to the R, G and B LEDs are adjusted such that the color temperature becomes equal to a predetermined specific value which represents white light. Further, the current values of current to be supplied to the R, G and B LEDs of each LED block are adjusted such that all of the LED blocks have an equal and maximum light emission luminance. Consequently, light emission by an equal color temperature and maximum luminance is implemented over the overall area of the backlight.
However, the backlight of the liquid crystal display apparatus disclosed in Patent Document 1 has a problem that power consumption is high and the contrast ratio of the display luminance is low. This is because, since the overall screen of the liquid crystal panel is usually illuminated with uniform and maximum luminance, for example, also in a case wherein a dark image is displayed, the backlight emits light with the maximum light emission luminance.
Therefore, in order to reduce the power consumption, a method has been proposed wherein not the overall screen of the liquid crystal panel is usually illuminated with uniform and maximum luminance but a lighting region of the backlight is divided into a plurality of regions and the light emission luminance for each region is varied in response to the luminance distribution of an image. The method is disclosed, for example, in Japanese Patent Laid-Open No. 2004-212503 (hereinafter referred to as Patent Document 2) or in Japanese Patent Laid-Open No. 2004-246117 (hereinafter referred to as Patent Document 3).